1. Field of the Invention
This invention relates to hose constructions and has particular reference to a hose suitable for use in transporting or otherwise handling refrigerant gases and fuel oils.
2. Description of the Prior Art
From the standpoint of environmental protection, there has been a strong demand for hoses of high performance or fluid impermeability particularly for use in the transport of refrigerant and fuel fluids. To cope with this trend calls for several important qualities including high impermeation to freon or fluorocarbon gases and to hydrocarbon gases, sufficient proofness to moistures, adequate flexibility and good retention of physical properties even at elevated temperature and hence immunity from fluid leakage out of metal joints at opposed hose ends.
It is known that a permeation-resistant hose can be constructed with an inner or core tube formed of a nitrile or NBR rubber and an outer cover of a chloroprene or CR rubber. This hose is hereunder called a first hose for brevity. As disclosed for instance in U.S. Pat. No. 4,950,436, a more advanced type of hose has been reputed which has a combination core including an inner peripheral wall and an outer peripheral wall. The inner wall disposed for direct contact with a gaseous or liquid fluid is made from a polyamide resin for its good gas-impervious nature and the outer wall from a rubbery material. This double-cored hose is referred to as a second hose.
The second hose is further described in terms of its physical characteristics as compared to the first hose.
1. Gas Impermeation
The second hose has been contrived to inhibit the tendency of the first hose becoming leaky of gas. This quality avoids environmental pollution and in turn ozone depletion. Maintenance-free properties are also improved with a cycle of gas replacement of 10 years. The first hose has a gas leak of 20 to 25 gf/m/72 hr at 100.degree. C. and a gas exchange cycle of 2 years in the case of use of dichlorofluoromethane or CFC 12 as a refrigerant or coolant. To suit maintenance-free transport, the limit of gas leak should be not more than 5 gf/m/72 hr at 100.degree. C.
Trifluoromonofluoroethane or HFC 134a has of late been accepted as a good substitute for the CFC 12 gas. The substitute gas is less likely to erode the ozone layer. The above limit of gas leak applies to the use of HFC 134a.
2. Flexibility
The second hose is designed not only for coolant delivery but also for vibration absorption between a fuel engine and a car body in automotive vehicles. Flexibility is important to that end and should be smaller than 2.0 kgf in bending strength as is acceptable in the first hose.
3. Moisture Proofness
When a relatively large proportion of moisture penetrates through the cover and then the core into a flowing refrigerant, condensation will take place in the refrigerating system and thus damage the corresponding compressor. Moisture permeation should be held to an absolute minimum and set at or below one third as small as the first hose. To be more specific, 0.2 mgf/cm.sup.2 /day at 50.degree. C. or smaller is acceptable as against 0.6 mgf/cm.sup.2 /day at 50.degree. C. peculiar to the first hose.
4. Freedom from Fluid Leak out of Metal Joints
The second hose is usually provided at both ends with metal joints clamped for fitting interengagement with the corresponding system. Stress retention should be as great as possible at high temperature. Initial stresses at or near the clamped joints will decline upon exposure of the hose to heat during its use. This causes a sharp drop in residual stress and a fluid leak from and around the hose-to-metal joint.
The second hose has its inherent advantages as it meets with the foregoing physical characteristics. Owing to its double-layered core, however, the second hose needs to be produced with many different steps of processing and also with a vulcanizing step of rubber. This is tedious and costly.
Alternatively, it has been proposed to construct a hose with an inner tube of a single layer formed from a gas-impervious polyamide resin and an outer cover from a moisture-proof fluorine rubber. Although easy to form with a minimum of steps devoid of vulcanization, such single-cored hose is undesirable for practical purposes. It is not sufficiently flexible and prone to soften on application of heat and hence leak at or around the associated metal fittings.